Polymetaphenylene isophthalamide-based polymer porous film, process for its production and battery separator

ABSTRACT

A polymetaphenylene isophtha lamide-based polymer porous film having a satisfactory porous structure that exhibits excellent gas permeability and heat resistance. It is produced by a process which comprises casting a dope of the polymetaphenylene isophthalamide-based polymer and coagulating it in a coagulating bath. The porous film may also contain inorganic whiskers, and a composite porous film may be formed in combination with a separate thermoplastic polymer film.

[0001] This is a Continuation of Application Ser. No. 10/030,247 filedJan. 9, 2002, which is a National Stage Application of PCT ApplicationNo. PCT/JP00/06234 filed Sep. 12, 2000; the above noted priorapplications are all hereby incorporated by reference.

TECHNICAL FIELD

[0002] The present invention relates to a heat resistant porous filmmade of a polymetaphenylene isophthalamide-based polymer, to a processfor production of the porous film and to a battery separator-comprisingthe porous film.

BACKGROUND ART

[0003] Polyolefin-based films, including those comprising polypropylene,have been known as porous films according to the prior art, but thesefilms lack heat resistance and when used at temperatures exceeding 180°C., for example, such films and their pores undergo considerabledimensional changes, leading to such problems as reduced function as aporous films.

[0004] Aromatic polyamides are known as alternative films with excellentheat resistance, and porous films of these polymers are described inJapanese Examined Patent Publication No. 59-14494 and No. 59-36939. Inrecent years, however, there has been a trend toward thinner porousfilms which has also led to a higher Young's modulus required for suchfilms; as the current processes have not allowed production of aromaticpolyamide porous films with adequate Young's moduli, it has beenattempted to improve the Young's modulus through orientation andcrystallization achieved by stretching. However, the conventionalprocesses have been associated with problems such as, for example,destruction of the porous structure due to softening at the highstretching temperatures for dry hot stretching, causing densificationand closing of the pores, or failure to achieve an increased draw ratiodue to the low temperatures of stretching in boiling water, such thatthe Young's modulus cannot be improved.

[0005] On the other hand, several different types of products have beendeveloped as sheets to be used as battery separators for lithium ionbatteries and the like, among which synthetic fiber nonwoven fabrics arewidely used because of their excellent gas permeability and impregnatingability and satisfactory mechanical properties (for example, JapaneseUnexamined Patent Publications No. 63-108664, No. 63-108665 and No.4-56062). Problems that typically occur with batteries includedeformation, extrusion of contents and ignition as a result ofovercharging or external shorting, and such problems have been counteredby adopting the use of highly heat resistant and chemically resistantaromatic polyamide porous films as separators for batteries, asdisclosed in the aforementioned Japanese Unexamined Patent PublicationsNo. 59-14494 and No. 59-36939. In addition, the use of nonwoven fabricsor sheets made of wholly aromatic polyamide (aramide) fibers as batteryseparators has been proposed in Japanese Unexamined Patent PublicationsNo. 5-335005, No. 7-78608 and No. 7- 37571. Nevertheless, while suchbattery separators exhibit lower internal electrical resistance and moreexcellent electrical properties with smaller thicknesses, it is verydifficult to industrially produce them to thicknesses of 50 μm orsmaller with adequate strength and with excellent uniformity, when suchfilms, nonwoven fabrics and sheets as described above are used.

[0006] A separator in a battery is an important member situated betweenthe positive electrode and the negative electrode, and recent years haveseen modifications designed to provide a shutdown function forseparators in order to meet the contradictory requirements of expectedelectrical properties and safety. Shutdown refers to cutting off thecurrent, due to closing of the pores as the porous film of the separatormelts upon increased battery temperature, that occurs in the event oftroubles such as overcurrent or external shorting. For this purpose,Japanese Unexamined Patent Publications No. 60-52 and No. 60-136161 haveproposed using supports consisting of porous films made of polyethyleneor polypropylene that allow manufacture of such thin porous films, andincluding a low melting point obstructive material capable of meltingwith heat, such that at high temperatures the obstructive material meltsand plugs the pores of the porous film to provide a shutdown function.However, since such materials employ polyethylene or polypropylenethermoplastic films as the base materials, the heat resistance is lowand there are many restrictions on their use from a safety standpoint.

DISCLOSURE OF THE INVENTION

[0007] In light of these circumstances, it is an object of the presentinvention to provide an aromatic polyamide-based polymer porous filmwith excellent heat resistance, chemical resistance and dimensionalstability, as well as a high Young's modulus and sufficient gaspermeability.

[0008] It is another object of the invention to provide a process forthe production of the aromatic polyamide-based polymer porous film.

[0009] It is yet another object of the invention to provide a batteryseparator employing the aromatic polyamide-based polymer porous film.

[0010] It is still yet another object of the invention to provide acomposite film comprising the aromatic polyamide-based polymer porousfilm, and a battery separator employing it.

[0011] It is still yet another object of the invention to provide abattery employing the battery separator.

[0012] As a result of diligent research aimed at overcoming theaforementioned problems, the present inventors have completed thepresent invention upon finding that it is possible to obtain apolymetaphenylene isophthalamide-based polymer porous film (hereunderalso referred to simply as “a polyamide porous film”) with a highYoung's modulus using a polymetaphenylene isophthalamide-based polymer,by casting a dope prepared by dissolving the polymer in an amide-basedsolvent, coagulating it in a coagulating bath comprising an amide-basedsolvent containing a non-solvent for the polymer and then, optionally,subjecting it to specific treatment such as stretching.

[0013] The present invention therefore provides a polymetaphenyleneisophthalamide-based polymer porous film with a gas permeability of0.2-1000 ml/sec, and which retains at least 60% of its gas permeabilityafter heat treatment at 350° C. for 10 minutes, compared to beforetreatment.

[0014] The invention further provides a polymetaphenyleneisophthalamide-based polymer porous film having a porous structure witha porosity of 60-80%, a cross-sectional pore laminar coefficient of 2.5or greater, and a specific Young's modulus of 300-800 Km in at least onedirection.

[0015] The invention still further provides a polymetaphenyleneisophthalamide-based polymer porous film with a gas permeability of0.2-1000 ml/sec, and which retains at least 60% of its gas permeabilityafter heat treatment at 350° C. for 10 minutes compared to beforetreatment, while also having a porous structure with a porosity of60-80%, a cross-sectional pore laminar coefficient of 2.5 or greater,and a specific Young's modulus of 300-800 Km in at least one direction.

[0016] The invention still further provides a polymetaphenyleneisophthalamide-based polymer porous film containing inorganic whiskersand having a porosity of 10-80% and a specific Young's modulus of200-5000 Km in at least one direction.

[0017] The invention still further provides a process for the productionof a polymetaphenylene isophthalamide- based polymer porous film thatinvolves casting a dope prepared by dissolving a polymetaphenyleneisophthalamide-based polymer in an amide-based solvent and coagulatingit in a coagulating bath comprising an amide-based solvent containing anon-solvent for the polymer.

[0018] The invention still further provides a battery separatorcomprising a porous film according to the invention as described above.

[0019] The invention still further provides a porous film comprising atleast two layers including a polymetaphenylene isophthalamide-basedpolymer porous layer and a heat-melting thermoplastic polymer porouslayer.

[0020] The invention still further provides a battery separatorcomprising a composite porous film according to the invention and alithium ion battery employing the battery separator.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] According to the first aspect of the polymetaphenyleneisophthalamide-based polymer porous film of the invention, the film hasa gas permeability of 0.2-1000 ml/sec and preferably 0.4-200 ml/sec, andretains at least 60% and preferably at least 70% of its gas permeabilityafter heat treatment at 350° C. for 10 minutes, compared to beforetreatment. If the gas permeability is greater than 1000 ml/sec theYoung's modulus is reduced and the handling properties are impaired,while if it is less than 0.2 ml/sec the pores collapse forming a densestructure. The porous film may be evaluated as having adequate heatresistance if at least 60% of the gas permeability is retained afterheat treatment at 350° C. for 10 minutes.

[0022] According to the second aspect of the polymetaphenyleneisophthalamide-based polymer porous film of the invention, the film hasa porous structure with a porosity of 60-80% and preferably 65-75%, anda cross-sectional pore laminar coefficient of 2.5 or greater andpreferably 3-500. Maintaining a porosity of 60-80% is important for usesrequiring high strength and adequate permeability; a porosity of lessthan 60% results in a lack of porous properties, and a porosity ofgreater than 80% causes a lack of strength. If the cross-sectional porelaminar coefficient is 2.5 or greater, the porous structure of the filmcross-section may be evaluated as satisfactory. The porous film alsopreferably has a specific Young's modulus of 200-800 Km in at least onedirection, while a specific Young's modulus of 300-700 Km is preferredfor uses requiring high performance.

[0023] According to the third aspect of the polymetaphenyleneisophthalamide-based polymer porous film of the invention, it has a gaspermeability of 0.2- 1000 ml/sec and retains at least 60% of its gaspermeability after heat treatment at 350° C. for 10 minutes compared tobefore treatment, while also having a porous structure with a porosityof 60-80% and a cross-sectional pore laminar coefficient of 2.5 orgreater, and a specific Young's modulus of 200-800 Km in at least onedirection.

[0024] The polymetaphenylene isophthalamide-based polymer porous film ofthe invention preferably has a thickness of 1-10 μm. If the thickness isgreater than 10 μm the battery capacity density may be insufficient andthe ion conductivity and charge efficiency may be reduced, when it isused as a battery separator. On the other hand, if the thickness is lessthan 1 μm the electrolytic solution holding power and mechanicalstrength may be insufficient. The thickness is more preferably 2-6 μm.

[0025] A process for production of a polymetaphenyleneisophthalamide-based polymer porous film according to the invention willnow be explained.

[0026] According to the invention, a polymetaphenyleneisophthalamide-based polymer porous film may be obtained by casting adope prepared by dissolving the polymetaphenylene isophthalamide-basedpolymer in an amide-based solvent, and coagulating the cast dope in acoagulating bath comprising an amide-based solvent containing anon-solvent for the polymer.

[0027] Generally, for example, the dope is cast on a support and thecast dope is immersed in the coagulating bath while residing on thesupport. The coagulated film may then be rinsed with water if desired,and once the amide-based solvent and coagulating bath have been removed,it may be stretched in a stretching bath comprising an amide-basedsolvent containing a non-solvent for the polymetaphenyleneisophthalamide-based polymer, or it may be immersed in a bath comprisingan amide-based solvent containing a non-solvent for thepolymetaphenylene isophthalamide-based polymer, and finally rinsed withwater. In such cases, any of the aforementioned rinsing steps may beomitted, but since the solvent that is used remains in the porous filmand can significantly lower the softening temperature due to aplasticizing effect unless rinsing is carried out after the stretchingtreatment or immersion treatment, rinsing with water is usuallypreferred.

[0028] The polymetaphenylene isophthalamide-based polymer used for theinvention is a polymer having a structure obtained by polycondensationof a meta-aromatic diamine and a meta-aromatic dicarboxylic acid, butcopolymerizable components may be used for a portion thereof. That is,the polymetaphenylene isophthalamide-based polymer is typically apolymer composed mainly of repeating units represented by the followingformula:

[0029] although there is no limitation to such types of polymers. Thecopolymerizable components may be, as examples of amine components andcarboxylic acid components, para-aromatic diamines, para-aromaticdichlorides, aliphatic diamines, aliphatic dicarboxylic acids, alicyclicdiamines and alicyclic dicarboxylic acids.

[0030] Specifically, as meta-aromatic diamines there may be mentioned1,3-phenylenediamine, 1,6-naphthalenediamine, 1,7-naphthalenediamine,2,7-naphthalenediamine and 3,4′-biphenyldiamine. As meta-aromaticdicarboxylic acids there may be mentioned isophthalic acid,1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid and3,4-biphenyldicarboxylic acid.

[0031] As specific copolymerizable monomers there may be mentioned, aspara-aromatic diamines, paraphenylenediamine, 4,4′-diaminobiphenyl,2-methyl-paraphenylenediamine, 2-chloro-paraphenylenediamine and2,6-naphthalenediamine; as para-aromatic dicarboxylic dichlorides,terephthalic chloride, biphenyl-4,4′-dicarboxylic chloride and2,6-naphthalenedicarboxylic chloride; as aliphatic diamines,hexanediamine, decanediamine, dodecanediamine, ethylenediamine andhexamethylenediamine; and as aliphatic dicarboxylic acids,ethylenedicarboxylic acid and hexamethylenedicarboxylic acid. However,these are not limited to those given above.

[0032] The polymer concentration of the dope comprising thepolymetaphenylene isophthalamide-based polymer of the inventiondissolved in an amide-based solvent (hereunder also referred to simplyas “dope”) is preferably 3-30 wt %, more preferably 3-25 wt % and mostpreferably 5-20 wt %.

[0033] As useful amide-based solvents for the dope there may bementioned such polar solvents as N-methyl-2-pyrrolidone,N,N-dimethylacetamide and N,N-dimethylformamide, but there is nolimitation. So long as the object of the invention is not hindered, thesolvents may be any that contain an amide group and dissolves thepolymetaphenylene isophthalamide-based polymer that is used. Becausesolvents that can dissolve the polymetaphenylene isophthalamide-basedpolymer used for the invention are limited to amide-based solvents, thesolvent used for the invention is limited to an amide-based solvent.

[0034] A monovalent or divalent cationic metal salt may also be used inorder to enhance the solubility of the polymetaphenyleneisophthalamide-based polymer. As a general rule it is preferred for sucha metal salt to be absent for the purpose of the invention, but if it isused, the metal salt is preferably present in the amide-based solvent inan amount of 0-50 parts by weight per 100 parts by weight of thepolymetaphenylene isophthalamide-based polymer, and as specific metalsalts there may be mentioned calcium chloride, lithium chloride, lithiumnitrate and magnesium chloride. The addition and dissolution of themetal salt in the amide-based solvent may be accomplished by aconventional method and may be done before, during or after dissolutionof the polymetaphenylene isophthalamide-based polymer.

[0035] The dope is cast onto the support, and the cast dope isintroduced into the coagulating bath together with the support. Here,the support may be a metal drum, an endless metal belt or an organicfilm, such as one made of polypropylene, polyethylene, polyethyleneterephthalate or the like. It is preferably one that has been subjectedto release treatment with silicone or the like.

[0036] The temperature of the dope for the casting is not particularlyrestricted, but it is preferably selected so that the viscosity of thedope is between 1 and 2000 poises, and more preferably between 5 and 500poises.

[0037] In order to maintain a sheet form for the cast dope duringcasting, it is effective to carry out the process of the invention byselecting the temperature range for the support and the atmospheresurrounding the support or by adjustment of the atmosphere surroundingthe support by air blowing, etc., but these conditions may be determinedby trial and error.

[0038] The coagulating bath contains an amide-based solvent and anon-solvent for the polymetaphenylene isophthalamide-based polymer. Thenon-solvent for the polymetaphenylene isophthalamide-based polymer ispreferably inactive with respect to the amide-based solvent.

[0039] As specific amide-based solvents there may be mentionedN-methyl-2-pyrrolidone, N,N-dimethylacetamide and N,N-dimethylformamide,among which N-methyl-2-pyrrolidone is preferred.

[0040] As non-solvents for the polymetaphenylene isophthalamide-basedpolymer there may be mentioned lower alcohols and lower ethers, althoughit is preferred to use water. Mixtures thereof may also be used.

[0041] The amide-based solvent to be used for the coagulating bath maybe the same as or different from the amide-based solvent used for thedope, but using the same solvent is advantageous since it allowsrecovery for reuse and reduces time and effort required for disposal.

[0042] A metal salt may also be added to the coagulating bath in anamount of 1-10 wt % with respect to the total coagulating bath, for thepurpose of adjusting the pore size of the resulting porous film. Asspecific examples of such metal salts there may be mentioned calciumchloride, lithium chloride, lithium nitrate and magnesium chloride.

[0043] The concentration of the amide-based solvent in the coagulatingbath is preferably 30-80 wt % and more preferably 50-70 wt % based onthe total coagulating bath. The temperature of the coagulating bath ispreferably 0-98° C. and more preferably 20-90° C.

[0044] If the concentration of the amide-based solvent in thecoagulating bath is less than 30 wt % or the temperature of thecoagulating bath is below 0° C., the number of pores on the surface ofthe resulting polyamide porous film will be reduced while the pore sizewill be smaller, tending to give a polyamide porous film with low gaspermeability. If the concentration is over 80 wt % or the temperature isabove 98° C., the polymer may become granular making it impossible toobtain a polyamide porous film. If either of the temperature andconcentration are outside of the aforementioned ranges, disadvantages interms of use will be presented, though not as much as when both areoutside the specified ranges.

[0045] The cast dope, which is the coagulated porous film, may also besubsequently rinsed and dried if desired.

[0046] The water rinsing is a treatment that halts coagulation andcrystallization of the porous film by removing the solid component andliquid components adhering to the porous film. Without adequate rinsing,fine solid components may remain adhered to the porous film, producingscum during the hot stretching. Residual liquid components aredisadvantageous in terms of cost and can produce unpleasant odors duringhot stretching, or decompose, or plug the pores due to the plasticizingeffect of the residual amide-based solvent or coagulating solution.However, it will usually not be necessary to carry out very thoroughrinsing in order to halt coagulation and crystallization of the porousfilm.

[0047] The drying may be carried out to any desired extent, and normallythis will involve anywhere from drying by nip roll treatment to anextent known as “hydro-extraction”, to more extensive drying with a hotair drier or the like. However, in order to facilitate setting of theconditions for the subsequent hot stretching and to obtain stableresults, it is preferred to keep the drying temperature within aprescribed range, for which reason the extent of drying is preferably toa moisture content of no greater than 100 parts by weight, morepreferably no greater than 30 parts by weight and most preferably nogreater than 5 parts by weight, per 100 parts by weight of theabsolutely dry porous film.

[0048] If desired, the resulting polyamide porous film may then besubjected to stretching in a stretching bath.

[0049] First, the polyamide porous film is immersed and plasticized in astretching bath comprising an amide-based solvent containing anon-solvent for the polymetaphenylene isophthalamide-based polymer, andis stretched therein.

[0050] As examples of useful amide-based solvents for the stretchingbath there may be mentioned N-methyl-2-pyrrolidone,N,N-dimethylacetamide and N,N-dimethylformamide, of whichN-methyl-2-pyrrolidone is preferred.

[0051] As non-solvents for the polymetaphenylene isophthalamide-basedpolymer there may be mentioned lower alcohols and lower ethers, althoughit is preferred to use water. Mixtures thereof may also be used.

[0052] The concentration of the amide-based solvent in the stretchingbath is preferably 5-70 wt % and more preferably 30-65 wt % based on thetotal stretching bath. The temperature of the stretching bath ispreferably 0-98° C. and more preferably 30-90° C.

[0053] If the concentration of the amide-based solvent in the stretchingbath is less than 5 wt % or the temperature of the stretching bath isbelow 0° C., the polyamide porous film may have insufficient plasticity,and the expected Young's modulus may not be achieved. If theconcentration is over 70 wt % or the temperature is above 98° C.,dissolution of the polyamide porous film may proceed making itimpossible to improve the Young's modulus by stretching, while theporous structure may collapse leading to greater density such that apolyamide porous film cannot be obtained.

[0054] The amide-based solvent used in the stretching bath may be thesame as or different from the amide-based solvent used for the dopeand/or the amide-based solvent used for the coagulating bath, but usingthe same solvent is advantageous since it reduces time and effortrequired for recovery and reuse or disposal.

[0055] The non-solvent for the polymetaphenylene isophthalamide-basedpolymer used in the stretching bath may also be the same as or differentfrom the non-solvent for the polymer used in the coagulating bath, butusing the same non-solvent is advantageous since it reduces time andeffort required for recovery and reuse or disposal.

[0056] The stretching method may be any method such as uniaxialstretching, successive biaxial stretching, simultaneous biaxialstretching or the like. For the stretching, reduction in the gaspermeability can preferably be avoided by holding and restraining bothsides in the direction of stretching.

[0057] The stretch ratio is preferably 1.3-5 in the uniaxial directionor 1.3-10 in the orthogonal biaxial directions, in order to achieve asuitable balance between the porosity, gas permeability and Young'smodulus. In the case of biaxial stretching, the stretching ratio of1.3-10 may be determined as the product of the stretching ratios in bothdirections (the area ratio).

[0058] The stretching may be carried out continuously from thecoagulating bath.

[0059] After stretching, the polyamide porous film may preferably beintroduced into water for rinsing, and then dried.

[0060] Alternatively, if desired, the stretching may be followed byimmersion of the resulting polyamide porous film in a bath comprising anamide-based solvent containing a non-solvent for the polymetaphenyleneisophthalamide-based polymer, to promote crystallization.

[0061] As examples of useful amide-based solvents for the immersion baththere may be mentioned N-methyl-2-pyrrolidone, N,N-dimethylacetamide andN,N-dimethylformamide, among which N-methyl-2-pyrrolidone is preferred.

[0062] As non-solvents for the polymetaphenylene isophthalamide-basedpolymer there may be mentioned lower alcohols and lower ethers, althoughit is preferred to use water. Mixtures thereof may also be used.

[0063] The concentration of the amide-based solvent in the immersionbath is preferably 50-80 wt % and more preferably 60-70 wt % based onthe total immersion bath. The temperature of the immersion bath ispreferably 50-98° C. and more preferably 60-90° C.

[0064] If the concentration of the amide-based solvent in the immersionbath is greater than 80 wt %, dissolution of the polyamide porous filmmay occur leading to breakdown of the porous structure, and if it isless than 50 wt %, crystallization may not proceed adequately. Also, ifthe temperature of the immersion bath is below 50° C., crystallizationof the polyamide porous film may not proceed or it may proceed withdifficulty, and if it is above 98° C., dissolution of the polyamideporous film may occur resulting in breakdown of the porous structure.

[0065] The amide-based solvent used in the immersion bath may be thesame as or different from the amide-based solvent used for the dopeand/or the amide-based solvent used for the coagulating bath, but usingthe same solvent is advantageous since it reduces time and effortrequired for recovery and reuse or disposal.

[0066] The non-solvent for the polymetaphenylene isophthalamide-basedpolymer used in the immersion bath may also be the same as or differentfrom the non-solvent for the polymer used in the coagulating bath, butusing the same non-solvent is advantageous since it reduces time andeffort required for recovery and reuse or disposal.

[0067] The immersion may be carried out continuously from thecoagulating bath.

[0068] After immersion, the polyamide porous film may preferably beintroduced into water for rinsing, and then dried. The rinsing anddrying are preferably carried out by the same methods as described forthe rinsing and drying after the coagulating treatment.

[0069] The polyamide porous film obtained after immersion preferably hasan insoluble portion of at least 10% in dimethylformamide (DMF) at 20°C.

[0070] The polyamide porous film may also be subjected to heat treatmentafter the immersion. The heat treatment is preferably carried out at atemperature of 290-380° C., and more preferably a temperature of330-360° C. The heat treatment is carried out for the purpose ofcrystallization, and a temperature of below 290° C. may result in aninadequate effect while a temperature of above 380° C. may causedecomposition of the polymer.

[0071] While heat treatment can sometimes reduce the porosity of theresulting porous film or plug the pores thus impairing the gaspermeability, there is no such effect with the porous film of theinvention, or at least such an effect is reduced to a minimum.

[0072] If desired, the aforementioned coagulating treatment may befollowed by rinsing and drying of the resulting polyamide porous film,and then hot stretching.

[0073] That is, the polyamide porous film obtained by coagulatingtreatment is then rinsed and dried by the methods described above, andthe polyamide porous film is then subjected to hot stretching. Thestretching temperature is suitable at 270-340° C., and more preferably290-320° C. If the stretching temperature is below 270° C. the porousfilm will have a lower stretch ratio and be susceptible to breaking,while if the stretching temperature is above 340° C. the porousstructure may collapse or the pores thereof may be plugged, leading togreater density. The heating may be accomplished by either a contactsystem or non-contact system, but it should be carried out as uniformlyas possible.

[0074] The stretching method may be any method such as uniaxialstretching, successive biaxial stretching, simultaneous biaxialstretching or the like. For the stretching, reduction in the gaspermeability can preferably be avoided by holding and restraining bothsides in the direction of stretching.

[0075] The stretch ratio is preferably 1.3-5 in the case of uniaxialstretching or 1.3-10 in the case of biaxial stretching, but it ispreferably selected in order to achieve a suitable balance between theporosity, gas permeability and Young's modulus. In the case of biaxialstretching, the stretching ratio of 1.3-10 may be determined as theproduct of the stretching ratios in both directions (the area ratio).

[0076] Alternatively, if desired, the immersion described above may befollowed by rinsing, drying and hot stretching of the obtained polyamideporous film. The rinsing temperature is not particularly restricted, buta greater rinsing effect is generally achieved by using hot water.

[0077] In this case, the immersion time is preferably 3-20 minutes whenthe temperature of the immersion bath is 50° C. or higher, since thisgives more suitable crystallization and helps to stabilize thesubsequent stretching conditions. The immersion time is more preferably5-15 minutes.

[0078] Next, the polyamide porous film is subjected to hot stretching.The stretching temperature is appropriately 270-380° C., and morepreferably 290-320° C. If the stretching temperature is below 270° C.the porous film may have a low stretch ratio and suffer breakage, whilea higher temperature above 380° C. may result in collapse of the porousstructure, plugging of the pores and a consequently higher density. Theheating may be accomplished by either a contact system or non-contactsystem, but it should be carried out as uniformly as possible.

[0079] The stretching method may be any method such as uniaxialstretching, successive biaxial stretching, simultaneous biaxialstretching or the like. For the stretching, reduction in the gaspermeability can preferably be avoided by holding and restraining bothsides in the direction of stretching.

[0080] The stretch ratio is preferably 1.3-5 in the case of uniaxialstretching or 1.3-10 in the case of biaxial stretching, but it ispreferably selected in order to achieve a suitable balance between theporosity, gas permeability and Young's modulus. In the case of biaxialstretching, the stretching ratio of 1.3-10 may be determined as theproduct of the stretching ratios in both directions (the area ratio).

[0081] According to the invention there is also provided apolymetaphenylene isophthalamide-based polymer porous film containinginorganic whiskers and having a porosity of 10-80% and a specificYoung's modulus of 200-5000 Km in at least one direction.

[0082] In this porous film, the weight ratio of the polymetaphenyleneisophthalamide-based polymer and the inorganic whiskers is 50:50 to 99:1and, preferably, the inorganic whiskers have a long axis dimension L of0.1-100 μm, a short axis dimension D of 0.01-10 μm and an L/D ratio of1.5 or greater.

[0083] The polyamide porous film may be produced by a process thatcomprises casting a dope prepared by dissolving the polymetaphenyleneisophthalamide-based polymer in an amide-based solvent and dispersingthe inorganic whiskers therein, and coagulating the dope in acoagulating bath comprising an amide-based solvent containing anon-solvent for the polymer.

[0084] This production process may be carried out using exactly the samematerials and in exactly the same manner as that described above forproduction of the polyamide porous film containing no inorganicwhiskers.

[0085] The inorganic whiskers used here may be anisotropic finefilamentous crystals and, as examples, there may be mentioned inorganicwhiskers of aluminum borate, potassium titanate, silicon carbide andsilicon nitride. The long axis dimension L of the inorganic whiskers ispreferably 0.1-100 μm and more preferably 1-20 μm while the short axisdimension D is preferably 0.01-10 μm and more preferably 0.05-2 μm; forthin-film purposes, the long axis dimension L is preferably 0.1-3 μm andthe short axis dimension D is preferably 0.01-0.5 μm. Also, the L/Dratio is preferably 1.5 or greater, more preferably 2.0-1000, and mostpreferably 5.0-200. The long axis dimension is the length of the longaxis, and the short axis dimension is the circle-equivalent diameter ofa cross-section of the short axis.

[0086] The method of mixing the inorganic whiskers into the dope may bea method whereby the inorganic whiskers are blended with a solution ofthe polymetaphenylene isophthalamide-based polymer dissolved in anamide-based solvent, or a method whereby the inorganic whiskers aredispersed in an amide-based solvent and then the polymetaphenyleneisophthalamide-based polymer is dissolved therein. The dispersion of theinorganic whiskers may be accomplished in a simple stirring tank, but itis preferably carried out using a ball mill, a kneader with a medium ofbeads, or a homomixer.

[0087] The weight ratio of the polymetaphenylene isophthalamide-basedpolymer to inorganic whiskers in the dope is 50:50 to 99:1, preferably60:40 to 95:5 and more preferably 70:30 to 90:10. If the proportion ofinorganic whiskers is greater than 50:50 the surface qualities of thefilm may be poorer, and if the proportion of whiskers is less than 99:1a sufficient effect may not be achieved.

[0088] The polymetaphenylene isophthalamide-based polymer porous filmobtained in this manner has a specific Young's modulus of 200-5000 Km inat least one direction and a porosity of 10-80% and, by includinginorganic whiskers, it can provide excellent heat resistance and ahigher Young's modulus, thus allowing application for various differentpurposes.

[0089] According to the invention, there is also provided a porous filmcomprising at least two layers including a polymetaphenyleneisophthalamide-based polymer porous layer and a heat-meltingthermoplastic polymer porous layer (hereunder also referred to as“composite porous film”).

[0090] The thermoplastic polymer used for the composite porous film ofthe invention is not particularly restricted, and as examples there maybe mentioned polyester resins such as polyethylene terephthalate, andaliphatic polyamide resins such as 2,6-nylon. These thermoplastic resinspreferably exhibit a “shutdown” property whereby they melt or deform,plugging the pores at a raised temperature, for example a hightemperature at which the battery produces abnormal heat, and thereforetheir heat deformation temperature (temperature at which pores begin tobe plugged due to heat contraction) is most preferably 60-150° C.

[0091] Polyolefins are preferred among these thermoplastic resins, andespecially ultrahigh molecular weight polyolefins with weight averagemolecular weights of preferably 400,000 or greater and more preferably800,000 or greater. As specific ultrahigh molecular weight polyolefinsthere may be mentioned polymers of ethylene, propylene, 1-butene,4-methyl-1-pentene, 1-hexene and the like, among which high density,ultrahigh molecular weight polyethylene is particularly preferred.

[0092] The simple method of producing a porous film made of a polyolefinis one in which the polyolefin solution is extruded through a die andthen cooled to obtain a gel composition which is then stretched to formpores, and examples of such methods are proposed in Japanese ExaminedPatent Publication No. 5-56251 and Japanese Unexamined PatentPublications No. 2-232242 and No. 3-64334.

[0093] The porous layer of the thermoplastic polymer preferably has athickness of 5-50 μm, more preferably 7-30 μm and most preferably 10-20μm. The porosity is preferably 30-70%, more preferably 40-65% and mostpreferably 50-60%.

[0094] On the other hand, the porous layer of the polymetaphenyleneisophthalamide-based polymer has a thickness of preferably 5-100 μm,more preferably 7-50 μm and most preferably 10-30 μm, and the porosityis preferably 30-80%, more preferably 40-70% and most preferably 50-65%.

[0095] A composite film may be produced by a process that comprisesforming a porous film made of a thermoplastic polymer as described aboveand, using this as a support, casting and coagulating a dope of apolymetaphenylene isophthalamide-based polymer onto one or both sidesthereof, as described in detail above for production of the polyamideporous film and the inorganic whisker-containing polyamide porous film.The materials and conditions for each step are, of course, exactly thesame as those described above.

[0096] The composite porous film obtained in this manner has a totalthickness of preferably 10-150 μm, more preferably 14-80 μm and mostpreferably 20-50 μm. The total porosity is preferably 30-70%, morepreferably 40-65% and most preferably 50-60%.

[0097] The composite porous film of the invention may also employ apolyvinylidene fluoride-based polymer as a heat melting thermoplasticpolymer. As examples of polyvinylidene fluoride-based polymers that areuseful for the invention there may be mentioned those described in U.S.Pat. No. 5,296,318 (copolymer of vinylidene fluoride andhexafluoropropylene), U.S. Pat. No. 5,571,634 (copolymer of vinylidenefluoride and chlorotrifluoroethylene) and Japanese Unexamined PatentPublication No. 9-289038 (copolymer of vinylidene fluoride and aperfluoroalkyl vinyl ether).

[0098] Specifically there may be mentioned polyvinylidene fluoride orcopolymers in which it is the main component, and as copolymercomponents that may be suitably used in such copolymers there may bementioned hexafluoropropylene, perfluoro lower alkyl vinyl ethers suchas perfluoromethyl vinyl ether, and chlorotrifluoroethylene, vinylfluoride, tetrafluoroethylene and the like; binary polymers andterpolymers of these copolymer components with vinylidene fluoride arealso suitable as polymer materials for the invention.

[0099] The method for production of a composite porous film with apolyvinylidene fluoride-based polymer porous layer is not particularlyrestricted, and one example is a method in which a porous film composedof a polymetaphenylene isophthalamide-based polymer is formed in themanner described above, and then a dope comprising a solution of thepolyvinylidene fluoride-based polymer (hereunder referred to as“fluorine-based dope”) is cast onto one or both sides thereof andcoagulated.

[0100] As examples of solvents for the fluorine-based dope there may bementioned amide-based solvents such as dimethylformamide,diethylformamide, dimethylsulfoxide, N-methyl-2-pyrrolidone,dimethylacetamide, diethylacetamide and hexamethylphosphoramide, whichare water-soluble aprotic polar solvents, but there is no limitation tothese.

[0101] A water-soluble phase separator may be added to thefluorine-based dope as a pore forming agent. As specific water-solublephase separators there may be mentioned polyethylene glycol with amolecular weight of 200-1000, ethylene glycol, diethylene glycol,triethylene glycol, polyethyleneglycol monomethyl ether with a molecularweight of 200-1000, glycerin, 1,4-butanediol, 1,3-butanediol, propyleneglycol, methanol, ethanol, isopropyl alcohol, 1-butanol, 2-butanol,tert-amyl alcohol, diacetone alcohol, ε-caprolactone, γ-butyrolactone,ethylene carbonate, propylene carbonate and triethyl phosphate; however,there is no limitation to these. These phase separators may be usedalone or in combinations of two or more thereof.

[0102] The polymer concentration in the fluorine-based dope ispreferably 3-30 wt %. At less then 3 wt % the mechanical properties ofthe film may be inadequate, and at greater than 30 wt % it may bedifficult to obtain a film with high porosity. The mixing proportion ofthe water-soluble solvent and the phase separator is preferably in therange such that the solvent/phase separator ratio is 10/0-5/5 (weightratio). If the proportion of the phase separator is greater than 5/5considerable gelling of the dope may occur, making it difficult to forma film and leading to inadequate mechanical properties of the resultingporous film.

[0103] The fluorine-based dope that is obtained is cast on thepolymetaphenylene isophthalamide-based polymer porous film and thenconveyed through a coagulating bath for coagulation.

[0104] The coagulating bath is one that is a good solvent for thepolyvinylidene fluoride-based polymer, and mixtures of amide-basedsolvents such as dimethylformamide, diethylformamide, dimethylsulfoxide,N-methyl-2-pyrrolidone, dimethylacetamide, diethylacetamide andhexamethylphosphoramide, which are water-soluble aprotic polar solvents,and non-solvents for polyvinylidene fluoride-based polymers such aslower alcohols, lower ethers and water, which are compatible with thesesolvents, may also be used; here, water is most preferred as thenon-solvent.

[0105] When a phase separator is added to the dope, the amide-basedsolvent is preferably used in the coagulating bath so that the ratio ofthe amide-based solvent/phase separator in the dope is the same.

[0106] The coagulating bath preferably has an amide-based solvent oramide-based solvent/phase separator mixed solvent concentration which isin a range of 5-70 wt % in total. If the concentration of theamide-based solvent or amide-based solvent/phase separator mixed solventis less than 5 wt %, a dense surface layer (skin layer) may be formed onthe coagulated surface, which can hamper impregnation of theelectrolytic solution during use as a battery separator, while if it ishigher than 70 wt % the coagulation time is prolonged, which can bedisadvantageous in terms of productivity. The temperature of thecoagulating bath is preferably in the range of 10-60° C.

[0107] The coagulated film obtained in this manner may then bepreferably rinsed and dried.

[0108] The porous film of the invention obtained by the method describedabove is suitable for such uses as filtration membranes for incineratorexhaust gases, bag filters or roll filters impregnated with cleaningfluids used for cleaning the ink remaining on copy machine transferdrums, which require high heat resistance and a high Young's modulus,and OA cleaners that require high heat resistance and strength; inaddition, these porous films are particularly useful as batteryseparators which require excellent heat resistance, chemical resistanceand dimensional stability as well as a high Young's modulus andsufficient porosity.

[0109] Consequently, the invention provides a battery separatorcomprising such porous films.

[0110] The battery separator of the invention has particularly excellentsurface uniformity and mechanical strength. It is also excellent as abattery separator because it exhibits excellent electrolytic solutionholding power and heat resistance and is particularly suited for use inbatteries with high capacity density, while also having excellent heatresistance, solution retention and mechanical properties, and highuniformity of thickness.

[0111] The battery separator of the invention may be used in a lithiumion battery according to the invention according to a known method, suchas any of the methods described in the examples provided below or themethod described in Japanese Unexamined Patent Publication No. 10-64503.

[0112] The present invention will now be further illustrated by way ofthe following examples and comparative examples which, however, are inno way intended to restrict the invention.

[0113] The various properties mentioned throughout the presentspecification were determined by the following methods.

[0114] [Gas Permeability]

[0115] The volume of air permeating per unit of time (ml/sec) wasdetermined according to the method of “JIS L1096-1990 6.27 gaspermeability”.

[0116] [Gas Permeability Retention]

[0117] The gas permeability is measured before and after heat treatmentin air at 350° C. for 10 minutes, and the gas permeability retention isdefined according to the following formula where T₀ ml/sec is the valueof the gas permeability before treatment and T ml/sec is the value aftertreatment.

Gas permeability retention=(T/T ₀)×100 (%)

[0118] [Porosity]

[0119] The dried porous film is cut to a size of A (mm)×B (mm), and thethickness C (mm) and weight D (g) are measured (A, B, C and D areappropriately selected). The apparent density E is determined accordingto the following formula.

Apparent density E=D/(A×B×C)×1000 (g/cm³)

[0120] The true density F of the polymer used is then determined, andthe porosity is calculated by the following formula.

Porosity=[(F−E)/F]×100 (%)

[0121] [Specific Young's Modulus]

[0122] The Young's modulus (kg/mm²) of the porous film as measured by atensile test is given as the value divided by the apparent density ofthe measured porous film.

[0123] [Cross-Sectional Pore Laminar Coefficient]

[0124] An SEM photograph of a cross-section of the porous film is taken,the pores are observed to an area 5 times the width in the lengthwisedirection with respect to the film thickness t, and the maximum lengthof the pore sizes in the cross-section in the direction of thickness isdefined as d. For example, when multiple circular pores are present inthe film cross-section, the maximum diameter is d.

[0125] The cross-sectional pore laminar coefficient is defined by thefollowing equation, where p is the porosity of the porous film.

Cross-Sectional Pore Laminar Coefficient=(t·p)/d

EXAMPLE 1

[0126] A dope comprising a mixed solution of polymetaphenyleneisophthalamide (relative viscosity=1.8) and N-methyl-2-pyrrolidone (NMP)in a weight ratio of 10:90 was cast onto a polypropylene film to athickness of 200 μm. This was immersed for 10 minutes in a coagulatingbath at 30° C. comprising 55 wt % NMP and 45 wt % water. The coagulatedproduct was then rinsed with water and dried for 30 minutes with a hotair drier at 130° C. to obtain a porous film.

[0127] The porosity of the resulting polyamide porous film was 70%, thegas permeability was 1.0 ml/sec and the cross-sectional pore laminarcoefficient was 39.2.

EXAMPLE 2

[0128] A dope comprising a mixed solution of polymetaphenyleneisophthalamide (relative viscosity=1.8), calcium chloride and NMP in aweight ratio of 9.5:4.5:86.0 was cast in the same manner as Example 1.This was immersed for 10 minutes in a coagulating bath at 30° C.comprising 60 wt % NMP and 40 wt % water. The coagulated product wasthen rinsed with water and dried for 30 minutes with a hot air drier at130° C. to obtain a porous film.

[0129] The porosity of the resulting polyamide porous film was 72%, thegas permeability was 1.3 ml/sec and the cross-sectional pore laminarcoefficient was 42.5.

EXAMPLE 3

[0130] The procedure of Example 1 was repeated, and after coagulation,the coagulated product was immersed for 10 minutes in a solution at 70°C. comprising 60 wt % NMP and 40 wt % water, and then rinsed with waterand dried to obtain a porous film. The porous film had a DMF insolubleportion of 60%.

[0131] The porosity of the resulting polyamide porous film was 73%, thegas permeability was 1.0 ml/sec and the cross-sectional pore laminarcoefficient was 37.8.

[0132] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 71% and the gas permeability was 0.8 ml/sec, andtherefore the gas permeability retention was 80%.

[0133] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 4

[0134] A porous film was obtained in the same manner as Example 1,except that the temperature of the coagulating bath was 80° C. Theporous film had a DMF insoluble portion of 90%.

[0135] The porosity of the resulting polyamide porous film was 69%, thegas permeability was 1.1 ml/sec and the cross-sectional pore laminarcoefficient was 42.8.

[0136] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 68% and the gas permeability was 1.0 ml/sec, andtherefore the gas permeability retention was 91%.

[0137] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 5

[0138] A porous film was obtained in the same manner as Example 4. Afterheat treatment of the porous film at 340° C. for 5 minutes in air theporosity was 68% and the gas permeability was 1.1 ml/sec, and thereforethe gas permeability retention was 98%.

[0139] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 6

[0140] The procedure of Example 1 was repeated to prepare a dope whichwas cast. This was immersed for 10 minutes in a coagulating bath at 30°C. comprising 60 wt % NMP and 40 wt % water. The coagulated product wasthen immersed for 5 minutes in a stretching bath at 50° C. comprising 50wt % NMP and 50 wt % water, and then stretched in the bath to a factorof 3 using a lateral restraining uniaxial stretching machine. Thestretched film was then rinsed with water and dried for 30 minutes witha hot air drier at 130° C. to obtain a porous film.

[0141] The porosity of the resulting polyamide porous film was 68%, thegas permeability was 2.0 ml/sec, the specific Young's modulus was 330 Kmin the stretching direction and 170 Km in the direction orthogonalthereto, and the cross-sectional pore laminar coefficient was 50.3.

[0142] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 7

[0143] The procedure of Example 6 was repeated, and the stretching wasfollowed by further stretching to a factor of 3 in the directionorthogonal to the first stretching direction. After then rinsing withwater, the 4 sides were attached to a metal frame and drying was carriedout for 30 minutes using a hot air drier at 130° C. to obtain a porousfilm.

[0144] The porosity of the resulting polyamide porous film was 65%, thegas permeability was 5.0 ml/sec, the specific Young's modulus was 320 Kmin the first stretching direction and 330 Km in the second stretchingdirection, and the cross-sectional pore laminar coefficient was 5.2.

[0145] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LIBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

Comparative Example 1

[0146] A dope comprising a mixed solution of polymetaphenyleneisophthalamide (relative viscosity=1.8), calcium chloride and NMP in aweight ratio of 15:0.3:85 was cast in exactly the same manner asExample 1. This was immersed in a coagulating bath consisting of waterat 50° C. and then immediately stretched to a factor of 1.2 and immersedfor 5 minutes. The coagulated product was then rinsed with water anddried for 30 minutes with a hot air drier at 130° C. to obtain a porousfilm.

[0147] The porosity of the resulting polyamide porous film was 85%, thegas permeability was 0.06 ml/sec and the cross-sectional pore laminarcoefficient was 1.3.

EXAMPLE 8

[0148] The porous film obtained in Example 1 was stretched to a factorof 2 in air while contacting it with a rod heater with a 5 mm diameterwhich had been heated to 320° C., to obtain an oriented porous film.

[0149] The porosity of the oriented polyamide porous film was 75%, thegas permeability was 2.5 ml/sec, the specific Young's modulus was 350 Kmin the stretching direction and 170 Km in the direction orthogonalthereto, and the cross-sectional pore laminar coefficient was 23.2.

[0150] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 71%, the gas permeability was 2 ml/sec, thespecific Young's modulus was 360 Km in the stretching direction and 180Km in the direction orthogonal thereto, and the cross-sectional porelaminar coefficient was 21.9. The gas permeability retention wastherefore 80%.

[0151] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 9

[0152] The porous film obtained in Example 3 was stretched to a factorof 2 in air while contacting it with a rod heater with a 5 mm diameterwhich had been heated to 360° C., to obtain an oriented porous film.

[0153] The porosity of the oriented polyamide porous film was 75%, thegas permeability was 2.9 ml/sec, the specific Young's modulus was 340 Kmin the stretching direction and 180 Km in the direction orthogonalthereto, and the cross-sectional pore laminar coefficient was 20.3.

[0154] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 61%, the gas permeability was 2.5 ml/sec, thespecific Young's modulus was 350 Km in the stretching direction and 190Km in the direction orthogonal thereto, and the cross-sectional porelaminar coefficient was 19.5. The gas permeability retention wastherefore 86%.

[0155] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 10

[0156] The porous film obtained in Example 4 was stretched to a factorof 2.5 in air while contacting it with a rod heater with a 5 mm diameterwhich had been heated to 360° C., and it was then stretched to a factorof 2.5 in the direction orthogonal to the first stretching direction, toobtain a biaxial oriented porous film.

[0157] The porosity of the oriented polyamide porous film was 78%, thegas permeability was 12.5 ml/sec, the specific Young's modulus was 330Km in the first stretching direction and 350 Km in the second stretchingdirection, and the cross-sectional pore laminar coefficient was 4.5.

[0158] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 76%, the gas permeability was 10.0 ml/sec, thespecific Young's modulus was 340 Km in the first stretching directionand 360 Km in the second stretching direction, and the cross- sectionalpore laminar coefficient was 4.1. The gas permeability retention wastherefore 80%.

[0159] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 11

[0160] A dope was prepared comprising an NMP solution containing 10 wt %polymetaphenylene isophthalamide (registered trademark: Cornex, byTeijin; relative viscosity: IV (in H₂SO₄)=1.8). The dope was then castonto a polypropylene film using a coater with a blade/film clearance of30 μm, and then coagulated for 5 minutes with a coagulating bath ofwater/NMP (45/55 weight ratio) at 80° C. The coagulated product wasrinsed with water at 80° C. and then dried with a hot air drier at 130°C. at a constant length to obtain a porous film. The porous film wascontacted with a stretching hot plate at 360° C. for stretching to afactor of 3 in the lengthwise direction to obtain an oriented porousfilm.

[0161] The oriented polyamide porous film had a thickness of 2.5 μm, aporosity of 63%, a gas permeability of 5 ml/sec, a specific Young'smodulus of 340 Km in the stretching direction and 180 Km in thedirection orthogonal thereto, and a cross-sectional pore laminarcoefficient of 4.0.

[0162] After heat treatment of the porous film at 350° C. for 10 minutesin air the porosity was 61%, the gas permeability was 4 ml/sec, thespecific Young's modulus was 500 Km in the stretching direction and 100Km in the direction orthogonal thereto, and the cross-sectional porelaminar coefficient was 3.2. The gas permeability retention wastherefore 80%.

[0163] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 12

[0164] Potassium titanate whiskers by Otsuka Chemical Co. (10 μm longaxis×0.5 μm short axis, density: 3.3 g/cm³) were dispersed in a solutioncomprising a mixture of polymetaphenylene isophthalamide andN-methyl-2-pyrrolidone (NMP) for casting, to prepare a dope with aninorganic whisker/polymetaphenylene isophthalamide/NMP ratio of 1/10/100by weight, and a whisker-containing polyamide porous film was obtainedin exactly the same manner as Example 1 except that the dope was castonto a polypropylene film to a thickness of 100 μm.

[0165] The polyamide porous film had a thickness of 30 μm, a porosity of62%, a gas permeability of 0.8 ml/sec, and a specific Young's modulus of600 Km in the stretching direction and 550 Km in the directionorthogonal thereto.

[0166] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 13

[0167] A whisker-containing polyamide porous film was obtained in thesame manner as Example 1, except that aluminum borate whiskers byShikoku Chemical Co. (20 im long axis×0.5 μm short axis, density: 3.0g/cm³) were used, the inorganic whiskers/polymetaphenyleneisophthalamide/NMP ratio of the dope used was 1/20/100 and the thicknessof the dope during casting was 70 μm.

[0168] The polyamide porous film had a thickness of 20 μm, a porosity of65%, a gas permeability of 0.4 ml/sec, a specific Young's modulus of 500Km in the stretching direction and 450 Km in the direction orthogonalthereto, and a cross-sectional pore laminar coefficient of 16.3.

[0169] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 14

[0170] A whisker-containing polyamide porous film was obtained in thesame manner as Example 11, except that the thickness of the dope duringcasting was 200 μm. It was then stretched to a factor of 3 in theuniaxial direction at a stretch rate of 20 mm/min by contact with a hotplate at 350° C.

[0171] The whisker-containing polyamide porous film had a thickness of22 μm, a porosity of 65%, a gas permeability of 2.0 ml/sec, and aspecific Young's modulus of 2500 Km in the stretching direction and 450Km in the direction orthogonal thereto.

[0172] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 15

[0173] A whisker-containing polyamide porous film was obtained in thesame manner as Example 12, except that the thickness of the dope duringcasting was 150 μm. It was then stretched to a factor of 3 in theuniaxial direction at a stretch rate of 20 mm/min by contact with a hotplate at 290° C.

[0174] The polyamide porous film had a thickness of 16 μm, a porosity of59%, a gas permeability of 0.8 ml/sec, a specific Young's modulus of1800 Km in the stretching direction and 400 Km in the directionorthogonal thereto, and a cross-sectional pore laminar coefficient of9.2.

[0175] The polyamide porous film was used as a separator in combinationwith a lithium cobaltate-based anode material, a carbonaceous cathodematerial and 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolyticsolution, to form a button battery with a diameter of 16 mm. The batterywas subjected to a 10-cycle test between 2.5-4.2 V with a constantcurrent of 1.0 mA, and the charge and discharge was satisfactorilyreproduced.

EXAMPLE 16

[0176] A dope comprising a mixed solution of polymetaphenyleneisophthalamide (relative viscosity=1.8) and N-methyl-2-pyrrolidone (NMP)in a weight ratio of 10:90 was cast onto an ultrahigh molecular weightpolyethylene porous film with a thickness of 20 μm and a porosity of 50%(registered trademark: Solpour, by Teijin DSM Soltech, K K.), to athickness of 100 μm, and then this was immersed for 10 minutes in acoagulating bath at 30° C. comprising 60 wt % NMP and 40 wt % water,rinsed with water and dried at 110° C. to obtain a composite porousfilm.

[0177] The composite porous film had a thickness of 50 μm, a porosity of60% and a cross-sectional pore laminar coefficient of 24.6 and hadcontinuous pores with a gas permeability of 0.30 ml/sec.

[0178] When the porous film was heated for 5 minutes at 170° C., thepores became plugged resulting in a gas permeability of substantially 0,thereby confirming that a shutdown property was exhibited. Nodeformation or contraction of the film itself was observed.

EXAMPLE 17

[0179] The procedure of Example 6 was repeated, and the stretching wasfollowed by further stretching to a factor of 3 in the directionorthogonal to the first direction of stretching. After then rinsing withwater, the 4 sides were attached to a metal frame and drying was carriedout for 30 minutes using a hot air drier at 130° C. to obtain a porousfilm.

[0180] The polyamide porous film had a porosity of 65%, a gaspermeability of 5.0 ml/sec, a specific Young's modulus of 320 Km in thefirst stretching direction and 330 Km in the second stretchingdirection, and a cross-sectional pore laminar coefficient of 3.7.

[0181] A PVDF copolymer comprising vinylidene fluoride andhexafluoropropylene in a molar ratio of 95/5 was dissolved in a mixedsolution comprising dimethylacetamide (DMAc) and polypropylene glycol(PPG) at a weight ratio of 60/40 to prepare a dope with a copolymerconcentration of 13 wt %, this was cast onto both surfaces of theaforementioned polyamide porous film to a thickness of 30 μm, and afterimmersion for 5 minutes in a coagulating bath at 30° C. comprisingDMAc/PPG/water in a ratio of 24/16/60 by weight, the film was rinsedwith water and dried for 30 minutes with a hot air drier at 80° C. toobtain a composite film. The thickness of the resulting composite filmwas 15 μm, the porosity was 58% and the cross-sectional pore laminarcoefficient was 10.9.

[0182] The composite film was used as a separator in combination with alithium cobaltate-based anode material, a carbonaceous cathode materialand 1 M LiBF₄PC/DEC (1/1 weight ratio) as the electrolytic solution, toform a button battery with a diameter of 16 mm. The battery wassubjected to a 10-cycle test between 2.5-4.2 V with a constant currentof 1.0 mA, and the charge and discharge was satisfactorily reproduced.

[0183] Industrial Applicability

[0184] According to the present invention it is possible to provide apolymetaphenylene isophthalamide-based polymer porous film whichmaintains its porous structure while exhibiting excellent heatresistance, chemical resistance and dimensional stability as well as ahigh Young's modulus and sufficient gas permeability, and is thereforesuitable for such uses as battery separator films that require excellentheat resistance, chemical resistance and dimensional stability as wellas a high Young's modulus and sufficient gas permeability, filtrationmembranes for incinerator exhaust gases, bag filters or roll filtersimpregnated with cleaning fluids used for cleaning the ink remaining oncopy machine transfer drums, which require high heat resistance and ahigh Young's modulus, and OA cleaners that require high heat resistanceand strength. As a process for production of such a polymetaphenyleneisophthalamide-based polymer porous film there is provided a processthat can easily accomplish stretching at low temperature withoutrequiring an expensive high-temperature tenter and that can provide apolyamide porous film with a high Young's modulus without destroying theporous structure.

1. A polymetaphenylene isophthalamide-based polymer porous film with agas permeability of 0.2-1000 ml/sec, which retains at least 60% of itsgas permeability after heat treatment at 350° C. for 10 minutes comparedto before treatment, while also having a porous structure with aporosity of 60-80%.
 2. A polymetaphenylene isophthalamide-based polymerporous film having a porous structure with a porosity of 60-80% and across-sectional pore laminar coefficient of 2.5 or greater, and having aspecific Young's modulus of 200-800 Km in at least one direction.
 3. Apolymetaphenylene isophthalamide-based polymer porous film with a gaspermeability of 0.2-1000 ml/sec, which retains at least 60% of its gaspermeability after heat treatment at 350° C. for 10 minutes compared tobefore treatment, while also having a porous structure with a porosityof 60-80% and a cross-sectional pore laminar coefficient of 2.5 orgreater, and having a specific Young's modulus of 200-800 Km in at leastone direction.
 4. A porous film according to any one of claims 1 to 3,which has a thickness of 1-10 μm and is self-supporting.
 5. Apolymetaphenylene isophthalamide-based polymer porous film containinginorganic whiskers and having a porosity of 10-80% and a specificYoung's modulus of 200-5000 Km in at least one direction.
 6. Apolymetaphenylene isophthalamide-based polymer porous film according toclaim 5, wherein the weight ratio of the polymetaphenyleneisophthalamide-based polymer to the whiskers is 50:50 to 99:1.
 7. Apolymetaphenylene isophthalamide-based polymer porous film according toclaim 5 or 6, wherein the inorganic whiskers have a long axis dimensionL of 0.1-100 μm, a short axis dimension D of 0.01-10 μm and an L/D ratioof 1.5 or greater.
 8. A process for the production of apolymetaphenylene isophthalamide-based polymer porous film, comprisingcasting a dope prepared by dissolving a polymetaphenyleneisophthalamide-based polymer in an amide-based solvent, and coagulatingit in a coagulating bath comprising an amide-based solvent containing anon-solvent for said polymer.
 9. A process according to claim 8, whereinthe concentration of the amide-based solvent in the coagulating bath is30-80 wt % and the temperature is 0-98° C.
 10. A process according toclaim 8 or 9, wherein the non-solvent for the polymetaphenyleneisophthalamide-based polymer is water.11. (previously presented): Aprocess according to claim 8, wherein the dope prepared by dissolving apolymetaphenylene isophthalamide-based polymer in an amide-based solventcontains no inorganic salts.
 11. A process according to claim 8, whereinthe dope prepared by dissolving a polymetaphenylene isophthalamide-basedpolymer in an amide-based solvent contains no inorganic salts.
 12. Aprocess according to claim 8, wherein after coagulation, the porous filmis rinsed with water, dried and then stretched to a factor of 1.3-5 inthe uniaxial direction or to a factor of 1.3-10 in the orthogonalbiaxial directions on an area scale, at a temperature of 270-340° C. 13.A process according to claim 8 wherein, after coagulation, the porousfilm is further stretched in a stretching bath comprising an amide-basedsolvent containing a non-solvent for the polymetaphenyleneisophthalamide-based polymer.
 14. A process according to claim 13,wherein the concentration of the amide-based solvent in the stretchingbath is 5-70 wt % and the temperature is 0-98° C.
 15. A processaccording to claim 8, wherein the coagulation is followed by immersionin a bath comprising an amide-based solvent containing a non-solvent forthe polymetaphenylene isophthalamide-based polymer, with an amide-basedsolvent concentration of 50-80 wt % and a temperature of 50-98° C.
 16. Aprocess according to claim 15, wherein the dimethylformamide-insolubleportion of the porous film after immersion is 10% or greater.
 17. Aprocess according to claim 15 or 16, wherein after the immersion theporous film is rinsed with water, dried and then heat treated at atemperature of 290-380° C.
 18. A process according to claim 15 or 16,wherein after the immersion the porous film is rinsed with water, driedand then stretched to a factor of 1.3-5 in the uniaxial direction or toa factor of 1.3-10 in the orthogonal biaxial directions on an areascale, at a temperature of 270-380° C.
 19. A process according to claim15 or 16, wherein after the immersion the porous film is furtherstretched in a stretching bath comprising an amide-based solventcontaining a non-solvent for the polymetaphenylene isophthalamide-basedpolymer.
 20. A process according to claim 19 wherein, after thestretching, the porous film is rinsed with water, dried and then heattreated at a temperature of 290-380° C.
 21. A process according to claim19, wherein the concentration of the amide-based solvent in thestretching bath is 5-70 wt % and the temperature is 0-98° C.
 22. Aprocess according to claim 8, wherein the dope used is one in whichinorganic whiskers are dispersed and a polymetaphenyleneisophthalamide-based polymer is dissolved in an amide-based solvent. 23.A process according to claim 22, wherein the weight ratio of thepolymetaphenylene isophthalamide-based polymer to the whiskers is 50:50to 99:1.
 24. A process according to claim 22 or 23, wherein theinorganic whiskers have a long axis dimension L of 0.1-100 μm, a shortaxis dimension D of 0.01-10 μm and an L/D ratio of 1.5 or greater.
 25. Aporous film comprising at least two layers including a polymetaphenyleneisophthalamide-based polymer porous layer and a heat-meltingthermoplastic polymer porous layer.
 26. A porous film according to claim25, wherein the thermoplastic polymer is a polyolefin with a molecularweight of 400,000 or greater.
 27. A porous film according to claim 25,wherein the thermoplastic polymer is a polyvinylidene fluoride-basedpolymer.
 28. A porous film according to claim 27, wherein thepolyvinylidene fluoride-based polymer is a copolymer composed mainly ofvinylidene fluoride and a perfluoro lower alkyl vinyl ether.
 29. Aporous film according to any one of claims 25 to 28 wherein, at elevatedtemperatures, the thermoplastic polymer layer melts and plugs the poregaps, while the polymetaphenylene isophthalamide-based polymer layerretains its shape without melting.
 30. A process for the production of aporous film which comprises forming a porous layer of apolymetaphenylene isophthalamide-based polymer onto one or both sides ofa porous film made of a heat-melting thermoplastic polymer, or forming aporous layer made of a heat-melting thermoplastic polymer onto one orboth sides of a porous film of a polymetaphenylene isophthalamide-basedpolymer.
 31. A battery separator comprising a porous film according toany one of claims 25 to
 28. 32. A lithium ion battery employing abattery separator according to claim
 31. 33. A method of using a porousfilm according to any one of claims 1-3 and 5-6 comprising placing saidporous film as a battery separator between a positive electrode and anegative electrode in a battery.
 34. A lithium ion battery comprising abattery separator situated between a positive electrode and a negativeelectrode, wherein said battery separator comprises a porous filmaccording to any one of claims 1-3 and 5-6.